The founder of Thailand's so-called yellow shirt protest movement which shut down Bangkok's airports last year was shot and wounded yesterdayon Thursday in an assassination attempt that his supporters said was politically motivated.

Sondhi Limthongkul suffered a head injury when his car was ambushed and riddled with bullets by at least two men in a pickup truck at a petrol station just before dawn, a spokesman for his People's Alliance for Democracy party said. A driver and bodyguard were also wounded, the driver seriously.

Chaiwan Chareonchoktawee, the director of Vajira hospital, which treated Sondhi, said some bullet fragments and bits of skull bone had been removed during surgery. "He is safe now and able to talk," the doctor said.

A spokesman for Sondhi's rightwing monarchist movement, supporters of which wore yellow shirts during last year's mass protests, said the was attack politically motivated.

"It is quite clear that it was political," the spokesman said, adding that the attack "did not come as a surprise". Sondhi was travelling at the time with two bodyguards, who were not wounded.

"We're not pointing the finger at anybody as the mastermind of this operation. We will wait for the government to find out," another PAD leader, Suriyasai Katasila, told the party's radio station.

Police said an investigation was under way to determine the motive. "Since he is a political figure, we will have to look into that," the police official in charge of the case said.

Bangkok's police commissioner, Vorapong Chiewpreecha, said 84 spent cartridges were found at the scene of the attack. He said: "We also found an M-79 grenade that was fired but missed Mr Sondhi's vehicle. It hit an empty public bus parked nearby but it did not go off."

The People's Alliance for Democracy staged protests throughout much of last year to demand that allies of the ousted prime minister Thaksin Shinawatra resign from government.

The yellow shirts were not behind the latest round of protests in Thailand, which involved their rivals, the red-shirted supporters of Thaksin, who rioted in Bangkok earlier this week but called off the protests on Tuesday after a military crackdown.

The Thai capital remained under emergency rule today for a sixth day. The state of emergency was imposed on Sunday, a day after Thaksin's supporters stormed a summit of Asian leaders outside Bangkok, prompting its cancellation and the evacuation of leaders by helicopter.